1. Field of the Invention
Aspects of the present invention relate to a positive electrode for a lithium secondary battery and a lithium secondary battery having the positive electrode, and more particularly, to a positive electrode for a lithium secondary battery capable of increasing a volume ratio of a positive electrode and maximizing a performance of the battery and a lithium secondary battery having the positive electrode.
2. Description of the Related Art
Recently, in the rapid development of electronic, communication, and computer industries, small-sized light-weight high-performance portable electric apparatuses such as camcorders, mobile phones, and notebook PCs have been widely used. Therefore, demands for batteries having a light weight, a long life cycle, and high reliability have increased. A lithium secondary battery has an operating voltage of 3.7 V or more, which is three times higher than that of a nickel cadmium battery or a nickel-hydrogen battery. In addition, lithium secondary batteries have a higher energy density per unit weight than nickel cadmium batteries or nickel-hydrogen batteries. Therefore, lithium secondary batteries have been used as a substitute for nickel cadmium batteries or nickel-hydrogen batteries in portable electronic apparatuses.
The lithium secondary battery generates electric energy through oxidation and reduction reactions while lithium ions are intercalated and de-intercalated at positive and negative electrodes. The lithium secondary battery is constructed by using positive and negative activation materials capable of reversibly intercalating and de-intercalating lithium ions and charging an organic or polymer electrolyte solution between the positive and negative electrodes.
Typically, lithium metal has been used as the negative activation material for the lithium secondary battery. However, when lithium metal is used, dendrites may be formed, and the battery may explode due to a short-circuit. Therefore, to replace the lithium metal, carbon-based materials such as amorphous carbon and crystalline carbon have been developed.
The positive activation material has the most important function for performance and safety of the lithium secondary battery. A chalcogenide compound may be used for the positive activation material. As an example thereof, research has been carried out on a composite metal oxide such as a composite of LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), and LiMnO2.
Among the positive activation materials, an Mn-based positive activation material has advantages in that it can be easily synthesized with low cost and generates a low level of environmental contaminants, but it has a disadvantage of a small capacitance. A Co-based positive activation material has advantages of a high electric conductivity, a high battery voltage, and excellent electrode characteristics, but it has a disadvantage of a high cost. A Ni-based positive activation material has advantages of the lowest cost and the highest discharge capacitance of the aforementioned positive activation materials, but it has a disadvantage in that it is not easy to synthesize.
In the recent research efforts to find better positive activation materials for the lithium secondary battery, much attention has been paid to finding a material that can be used as a substitute for LiCoO2 and that is capable of having stability at a high charge voltage of 4.2 or more, a high energy density, and a long life cycle. For example, LiCoO2, LiNiO2 derivative compounds obtained by changing compositions of Ni, Co, and Mn in the compounds LiNixCo1-xO2 (0<x<1), LiNixMn1-xO2 (0<x<1), and Li(NixCo1-2xMnx)O2 (0<x<1) have been developed (see Solid State Ionics, 57, 311 (1992), J. Power. Sources, 43-44, 595 (1993), Japanese Patent Application Publication No. H8-213015 (Sony (1996)), and U.S. Pat. No. 5,993,998 (Japan Storage Battery) (1997)). However, the positive activation materials obtained by simply changing the composition of Ni, Co, and Mn have not yet been found to be a good substitute for LiCoO2.
On the other hand, a high capacity battery using a positive electrode formed with high composite slurry has been proposed. The positive electrode is constructed by dispersing a positive electrode composite into a solvent such as N-methyl-2-pyrolidone to form a positive electrode composite slurry, coating the positive electrode composite slurry onto an aluminum foil, and drying the slurry. Generally, in order to increases the density of the composite, a rolling process is performed. If the density of the positive electrode composite is increased, the capacity per unit volume is increased, so that the capacity of the battery can be increased. However, when a rolling process is used, the activation material particles may be crushed or destroyed, depending on the particle sizes or types of the positive activation materials, and the composite layer may peel off or become detached. Therefore, it is difficult to increase the density of the positive electrode composite by a rolling process. In addition, since an electrolyte solution cannot easily permeate into a positive electrode composite layer that has a high density, charge and discharge characteristics may be degraded. In addition, in a course of charge and discharge cycles, the capacity of the battery may deteriorate.